Why did Celts of the British Isles build over 4000 hill forts during the Iron Age? The answer may have something to do with the conflict between those Bronze Age cowboys who maintained traditional pastoralist transhumance lifestyles, and those in the forts who depended more on arable farming. This can also explain why British Celts were the most lactose tolerant people on Earth at that time. In this documentary I visited Barbury Castle in Wiltshire, Castle an Dinas in Cornwall and several other magnificent Iron Age hill forts.